Radio frequency attenuator



NOV. 12, 1963 s, GlORDANo 3,110,872

RADIO FREQUENCY ATTENUATOR Filed. April 16. 1962 2 Sheets-Sheet 1 INSULATED ACTUATOR RAI FOR BAND 45 "I; im e TOOL G\ .1

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INVENTOR. SALVATORE GIORDANO BY Maw NOV. 12, 1963 s, GIORDANO 3,110,872

RADIO FREQUENCY ATTENUATOR Filed April 15. 1962 2 Sheets-Sheet 2 INJECTOR OUADRUPOLE l7 Focusme I5 MAGNET PROTON BUNCHER SUPPLY DEBUNCHER SOURCE PRE INJECTOR M.

k ALTERNATING GRADIENT SYNCHROTRON POWER SOURCE OUTPUT ET. H. RE AMP OUTPUT R F STORAGE NETWORK AND SPARK GAPS TO SUPPLY FREQUENCY CONTROL HE T EXCHAIIQVGER J 6| 45 3 r- 59 DRAIN :3

INVENTOR. 6 SALVATORE. GIORDANO BY flaw 4M...

United States Patent 3,110,872 RADIO FREQUENCY ATTENUATOR Salvatore Giordano, Port Jefferson Station, N.Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Apr. 16, 1962, Ser. No. 187,997 7 Claims. (Cl. 33381) This invention relates to radio-frequency control systems and more particularly to a radio-frequency attenuator for coaxial cables.

In many industrial and research apparatuses it has been necessary to control high peak, high-frequency energy in coaxial cables. One apparatus where such control has been necessary has been the Brookhaven high-energy particle accelerating injector described in The Linear Accelerator Injector for the AGS by S. Giordano, printed in the 1960 International Convention Record, Part 9. The radio-frequency power for this injector has been required to be attenuated, particularly for the injector buncher which has required a 200 megacycle, kw. peak, 20 watt average adjustably controlled attenuated input.

Heretofore, it has often been necessary when seeking to control or attenuate the intensity of radio-frequency signals being carried by a coaxial cable to use devices internal to the cable, or to use a discontinuous central conductor. As a consequence, it has further been necessary to use complicated impedance matching devices inside or along the cable which have introduced complexities involving relatively great weight and expense as well as difficulties in manufacturing the cables. Moreover, these and other prior art attenuators have been unsatisfactory for high peak power signals, for example, up to about 20 kw. or above. Additionally, it has been advantageous to provide an attenuator that has a low voltage standing wave ratio (VSWR), an attenuator that can readily be inserted along a cable between the ends of the cable without breaking the central conductor thereof, and an attenuator that can easily be adjusted forvariable energy attenuation in the cable. Also, it has been desirable to eliminate air gaps in attenuators known heretofore that have permitted corona discharge and it has been universally accepted that a simple or improved attenuator has been needed without complicated resistive and reactive elements.

During the search for .an improved attenuator for the above-mentioned injector buncher, it was discovered that principles of the coaxial cable termination described in co-pending application S. N. 78,193, filed December 23, 1960, and entitled Radio-Frequency Load by the inventor of this application, which is assigned to the assignee of this invention, might be useful for an attenuator even though the problems and solutions involved in coaxial cable terminations and attenuators have been different. In accordance with this co-pending application, a bell-shaped member has been connected to the end of the outer conductor of a coaxial cable and the cable insulation has been exposed around the central conductor to a liquid dielectric. It was found, however, that one or more of the members of this disclosure were adapted only for cable terminations or were otherwise not satisfactory for insertion along a coaxial cable between the ends of the cable for attenuation of the energy in the cable. For example, the VSWR could not be controlled separately and the ends of at least one of the members had to be modified to tie it in with an outer conductor.

In accordance with this invention a simple, high-peak power, radio-frequency attenuation system for a coaxial cable is provided that is operalble along a conventional coaxial cable between the ends thereof and provides a low voltage standing wave ratio (VSWR). More particularly, in one embodiment this invention contemplates a high frequency attenuator for a coaxial cable having an inner conductor, an outer annular conductor split to form spaced ends across a gap, an insulator between said conductors which is exposed at said gap, means contacting water with said insulator at said gap, 21 first metallic flare from said outer conductor at one side of said gap, a second metallic flare from said outer conductor at the opposite side of said gap, and an annular metallic conducting band around said insulator in said gap that is operable with said flares to counteract the tendency of said attenuator to produce a high insertion or reflection voltage standing wave ratio.

In another aspect, this invention provides for calorimetric measurements of the attenuation supplied by said attenuator.

The above and further novel features of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. 'It is to be expressly understood, lrowever, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

In the drawings where like parts are numbered alike:

FIG. 1 is partial cross-section of an embodiment of this invention;

FIG. 2 is partial cross-section of FIG. 1 through II-II;

FIG. 3 is partial cross-section of another embodiment of the radio-frequency attenuator of FIG. 1;

FIG. 4 is partial schematic drawing of a circuit for the attenuator of this invention;

FIG. 5 is schematic drawing of auxiliary components for the attenuator of this invention;

FIG. 6 is cross-section of another embodiment of the attenuator of FIG. 1.

Referring first to FIG. 4, conventional co-axial cable 11 carries radio-frequency energy from source 13 to drive a machine 15 such as an injector buncher for a linear accelerator 17 like the above-mentioned Brookhaven AGS Injector. The radio-frequency energy for such a buncher includes a 200 megacycle, 20 kw. peak, 20 watt average input, but other high peak sources 13 may supply other machines 15 in accordance with this invention through cable 11 when the supply needs control or attenuation, as will be understood in more detail hereinafter. Cable 11 includes a conventional, continuous solid, inner longitudinally extending copper conductor 19 of uniform cross-section; a conventional outer thin metallic lowpower loss factor conductor 21 such as a thin woven or braided copper conductor; and a conventional annular insulator 23 between conductors 19 and 21 such as is provided by polyethylene or other suitable low power loss factor insulators through which radio-frequency energy can pass without causing a substantial heating effect in the insulator. Advantageously, both the outer conductor and the insulator 23 have a power loss factor within the same limits.

, The ability of a material to undergo dielectric heating, or to accept high frequency energy may be explained in terms of the following equation:

wherein:

P., is the power absorbed per unit volume of material (the concentration of heat generated);

1 is the frequency in megacycles per second; e is the dielectric constant of the material (the ratio of the capacitive current through the material to the capacitive current which would flow if the same field intensity was applied to free space);

E is the electrical force in kilovolts per inch; and

Patented Nov. 12, 1963 a is the phase difference between the electrical force E and the power factor. The product of the dielectric constant e and the power factor" tan 6 is called the power loss factor. Materials such as water possessing a high power loss factor undergo dielectric heating. In contrast low power loss factor insulating materials including polyethylene and related polymeric materials such as polystyrene do not usually undergo substantial dielectric heating, it being theorized that such materials have a willcicntly general symmetry in their molecular structure that they are not heated dielectrically. As an illustration of the above effects, at a frequency of between and 100 megacycles per second, polystyrene has a power loss factor ranging from about 0.04 to 3. Polystyrene has been suitable for insulator 23 and other suitable low power loss, high dielectric materials for insulator 23 include glass, quartz, ceramics, polymethacrylates, polytetrafluorethylene, polyester-impregnated glass fibres and the like. On the other hand, vinyl chloride and vinyl acetate which are not suitable for the insulator 23 of this invention because they tend to overheat in a high frequency field have a power loss factor of greater than about 20 in the above mentioned frequency range.

The described coaxial cable 11 with polymeric (polyethylene or polystyrene) insulator 23 is conventional. The radio-frequency intensity attenuators, however, that have been available for such cable 11 heretofore, have required complicated apparatus inside the cable, have required a break in the inner conductor, complicated impedance matching devices, or have not been suitable for high peak currents up to 20 kw. or above. Additionally it has been advantageous to provide an improved attenuator that has a low VSWR, is adapted for use with conventional coaxial cable, can readily be inserted between the ends of conventional coaxial cable, is easily adjustable, has no air gaps that permit corona discharge and spark over between internal high voltage parts, that does not require complicated restive and reactive components, and that is simple.

In accordance with this invention, attenuator 25 is insertable along cable 11 between the ends 27 and 29 of the cable 11 for the attenuation of the high peak radiofrequency power in the cable 11 while providing a low VSWR of about 1.221. More particularly, this attenuator 25 contemplates a coaxial cable 11 having an inner conductor 19, an outer annular conductor 21 split to form spaced ends 31 and 33 across a gap 35, container 37, dielectric means 39 in said container and contacting said insulator 23 at said gap, a metallic first flare 41 from said outer conductor 21 at one side of said gap 35, an opposing metallic flare 43 from said outer conductor 21 on the opposite side of said gap 35, and an annular metallic band 45 around said insulator 23 in said gap 35 that is movable between said flares to adjust the voltage standing 'wave ratio of said attenuator from a relatively high value to a relatively low value, e.g. see FIG. 1.

Container 37 comprises a box having a metal lining 47 for shielding and a size to maintain a sufficient diameter of dielectric 39 around cable 11 of attenuator 25. A diameter, for example, of ten inches of dielectric has been found to be sufficient for the energy in cable 11 for the mentioned injector buncher application. Advantageously, suitable rubber gaskets 48 interposed between the cable 11 and the container 37 keep the dielectric 39 from leaking out of container 37 as shown in FIG. 1.

Dielectric 39 has a high power loss factor that enables it to be heated by radio-frequency energy that passes from inner conductor 19 through insulator 23 at gap 35. It has been found that water has a sufficiently high power loss factor for dielectric 39. Its power loss factor can be increased, however, by additives such as are well known.

Flares 41 and 43 may advantageously be formed in steps by cutting out tubular sections from outer conductor 21 to form band 45 and spaced ends 31 and 49 of conductor 21, the tubular section extending from end 31, to

end 49 and being slit longitudinally for removal from insulator 23. The ends 31 and 49 are flared by inserting a pointed tool 51 between the conductor 21 and insulator 23 at a point of beginning P and rotating the point 53 of the cable to the point of beginning P as illustrated in FIG. 1. Since the cable is Woven or braided the cable stitching gives sufficiently to form self-sustaining flares 41 and 43. Thereupon cutting of outer conductor 21 at a distance L" from the base of flare 41 forms ends 33 and 55 of conductor 21 and a sleeve 57 which is pulled up over end 33 of conductor 21 to form gap 35 between ends. 31 and 33 of conductor 21 in which the insulator 23 is exposed to dielectric 39. insulator handle 59 has a connection 61 with sleeve 57 for movement of flare 43 toward and away from flare 41 so as to change the amount of attenuation of the energy in cable 11. For example, the movement of movable flare 43 away from flare 41 up to a distance X of about 1 foot has changed the attenuation of the energy in cable 11 to the desired level. This attenuation causes heating of liquid 39 and this heating is an indication of the attenuation as will be described further hereinafter. In this example distance L is about two feet while the flares 41 and 43 have been about six inches long, each at a 45 angle to the axis of cable 11.

Annular metallic band 45 around insulator 23 in said gap 35 moves on insulator 23 toward and away from flares 41 and 43 to counteract the tendency of the attenuation of the energy in cable 11 to cause the insertion voltage standing wave ratio of the attenuator 25 to be high. Actual tests, for example, have shown that one described movement of band 45 relative to the flares 41 and 43 has effectively reduced the insertion VSWR of attenuator 25 from 2:1 to 1.2:1.

In making band 45, the band 45 may comprise a portion 45 of the section of outer conductor 21 which is removed to form ends 31 and 33 thereof as described above. Turning this remaining portion 45 will loosen it from insulator 23. The band 45, however, may also be a thin solid copper band or a like conductor with a low power loss factor and with a longitudinal slit from end to end for assembly around insulator 23.

In the operation of this invention water fills into container 37 from a suitable source (for example as shown in FIG. 5) and flare 43 moves toward or away from flare 41 to adjust the attenuation of the energy in cable 11 to the desired level. Movement of flare 43 toward flare 41 reduces the attenuation, and movement of flare 43 away from flare 41 increases the attenuation of the energy in cable 11 by, respectively decreasing or increasing the heating of dielectric 39. This heating may be measured by a suitable thermometer 63 which normally reaches a steady state condition after the'movement of flare 43 to a given position. Also, moving the flare 43 will change the attenuation of the energy in cable 11 in a direction up or down as indicated by the change in temperature of dielectric 39 respectively up or down and the amount of change of the temperature at the dielectric 39 will correspond to the relative attenuation of the energy in cable 11 by attenuator 25. Separate movement of band 45 changes the VSWR.

In another embodiment of the container 37 of this invention shown in FIG. 3 fixed flare 71 (corresponding to flare 41 shown in FIG. 1) has an extension 73 attached thereto to form a container 75 like container 37 only the former is adapted more easily to be placed inan upright position to save weight and space when it is desired to run cable 11 vertically. In this embodiment the extension 73 comprises a tubular low power loss material which may be thinly metal lined for shielding and which is easily attached to flare 71, for example, by an epoxy resin such as Epon 828, Bakelite ERL2774, Dow Resin X-2633.4, Epiphen Epoxides, Asaldite 502, EpiRez 510 or polyglycidal novolak resins or diapoxides such as RDGE. Suitable materials for extension 73 include glass, quartz, ceramics, polymethacrylates, polytetrafluorethylene, polyester-impregnated glass fibres and the like. The other elements, including cable '77, insulator 79 and flare 81 correspond with the above-described elements 11, 23 and 43, respectively.

It is understood that, the utility of the attenuator of this invention is not limited to use with the above-mentioned injectors but its use extends to coaxial cables supplying high-frequency power to many different kinds of machines for many different kinds of applications where the mentioned high peak power is required.

It is also understood that, the attenuation of this invention can be changed by changing the power loss factor of the liquid contacting the cable insulator and into which high frequency power is absorbed by dielectric heating, or as it is sometimes called, resistance heating. For example, as illustrated in FIG. 5, the mole ratio of soluble additives including sodium chloride or insoluble solid 7 additives such as clay to the water can be increased or decreased in a suitable manner so that the dielectric 39 absorbs more or less energy without moving the flares connected to the outer cable conductor. In this embodiment the insulated handle 59 for flare 43 and the connection 61 may be optional.

It is additionally understood that, flares 41 and 4L3, and band 45 are advantageously made from outer woven or braided conductor 21 since this conductor 21 has a low power loss factor, but one or more spaced metallic bands such as the thin solid copper bands 91, 92 and 45 with and without flares as is shown can also be slipped onto outer conductor 21, insulator 23 as shown in FIG. 6.

This invention provides a simple adjustable attenuator for the energy flowing in a coaxial cable carrying high peak, high-frequency power and provides a very low insertion VSWR. Moreover, this invention has the advantages of use with conventional longitudinally extending coaxial cable, easy insertion between the ends of the cable while retaining its continuous central conductor and inexpensive parts which eliminate the need for complicated resistive and reactive components or air gaps which permit corona discharge and spark over.

I claim: I

1. A radio-frequency attenuator for a coaxial cable having inner and outer conductorsseparatedby an an nular insulating element in which a prepared central portion of said cable has the outer conductor removed to expose a first portion of said insulator for a predetermined length, said outer conductor also being removed to form a flared first end portion of said outer conductor and a cylinder second end portion of said outer conductor which is spaced from said first end portion and between which said insulator is exposed, a tank containing water in contact with said exposed first portion of said insulator, a sleeve slidably engageable on said first cylindrical end and having a flared second end portion opposing said flared first end portion of said outer conductor and being movable slidably along the axis of said first conductor whereby energy in said first conductor passes through said insulator into said water selectively to attenuate high frequency energy in said cable an amount corresponding to the movement of said sleeve over said first cylindrical end of said outer conductor, and an annular conductor around said insulator between said flared first portion of said outer conductor and said flared second portion of said sleeve and which is slidably movable along the axis of said first conductor in operable association with said flared portions to adjust the voltage standing wave ratio along said attenuator to about 1.2: 1.

2. The invention of claim 1 in which said tank includes a low power loss factor member which extends vertically from said flared first end portion of said outer conductor and overlaps said sleeve for holding said water therein in contact with said insulator between said flared first end portion and said sleeve whereby said coaxial cable can be vertically disposed in a confined space.

3. A radio-frequency attenuator for a coaxial cable having an inner conductor, an outer annular conductor split to form spaced ends across a gap, and an insulator between said conductors which is exposed at said gap, a container, dielectric means therein contacting said insulator at said gap, a metallic first flare from said outer conductor at one side of saidv gap, a metallic second flare from said outer conductor at the oppositeside of said gap, and an annular metallic band around said insulator in said gap that is movable between said flares to counteract the tendency of said attenuator to produce a high voltage standing wave ratio.

4. The invention of claim 3 in which at least one of said first and second flares is slidably engageable with said outer conductor, to adjust the attenuation of radio-frequency energy in said cable, and said annular metallic band is slidably engageable with said insulator to adjust the voltagestanding wave ratio of said attenuator to about 1.2: 1.

5. The invention of claim 3 in which said first means has a watertight, low-power loss factor container which extends from said first flare and overlaps said second means for holding water therein in contact with said insulator.

6. A radio-frequency attenuator for a coaxial cable having an inner metallic conductor, an outer annular low power loss conductor with a split that forms outer conductor ends spaced by a gap, and a low power loss factor insulator between said conductors which is exposed, a container having a high dielectric, high power loss factor liquid therein which is exposed to said insulator at said gap, opposing metallic flares connected to said outer conductor at said spaced ends, and a metallic band around said insulator in said gap that is operable to minimize the voltage standing wave ratio of said attenuator.

7. The invention of claim 6 in which said flares are slidably engageable along said outer conductor across said spaced outer conductor ends to adjust the attenuation of the energy in said cable, and said metallic band is slidably engageable along said insulator to adjust the voltage standing wave ratio of said attenuator.

References Cited in the file of this. patent UNITED STATES PATENTS Chin et a1. July 10, 1962 

6. A RADIO-FREQUENCY ATTENUATOR FOR A COAXIAL CABLE HAVING AN INNER METALIC CONDUCTOR, AN OUTER ANNULAR LOW POWER LOSS CONDUCTOR WITH A SPLIT THAT FORMS OUTER CONDUCTOR ENDS SPACED BY A GAP, AND A LOW POWER LOSS FACTOR INSULATOR BETWEEN SAID CONDUCTORS WHICH IS EXPOSED, A CONTAINER HAVING A HIGH DIELECTRIC, HIGH POWER LOSS FACTOR LIQUID THEREIN WHICH IS EXPOSED TO SAID INSULATOR AT SAID GAP, OPPOSING METALLIC FLARES CONNECTED TOSAID OUTER CONCONDUCTOR AT SAID SPACED ENDS, AND A METALLIC BAND AROUND SAID INSULATOR IN SAID GAP THAT IS OPERABLE TO MINIMIZE THE VOLTAGE STANDING WAVE RATIO OF SAID ATTENUATOR. 